Drilling fluids are reputed as the blood for bore wells. Good drilling fluid techniques are one of the important guarantees for safe, high-quality, efficient, and quick well drilling production. Drilling fluids are required in the development of oil fields.
Effective reservoir protection is an effective guarantee for improving the rate of oil and gas recovery, so that the skin factor can be an optimal negative value. If the drilling fluid is designed or used inappropriately in the well drilling process, the liquid and solid in the drilling fluid may intrude into the oil reservoir and have physical and chemical reactions with clay and other minerals in the oil and gas layer. Consequently, the permeability in the oil layer in the immediate vicinity of the well may be decreased severely, and the resistance against air and gas flow towards the bottom of the well may be increased, resulting in reduced oil yield.
Usually, the permeability of middle permeability reservoirs is 70-200 mD. Since the permeability of middle permeability reservoirs is not as low as that of low-permeability reservoirs and is not as high as that of high-permeability or super-high-permeability reservoirs, drilling fluids that are designed for low-permeability, high-permeability, or super-high-permeability reservoirs may not be suitable for protection of such middle permeability reservoirs. Middle permeability reservoirs are also confronted with damages in many aspects, for example, blockage resulted from the solid particles carried by the drilling fluid and particles produced in the drilling process, reservoir damages resulted from hydrated expansion, dispersion and migration of clay minerals; the intrusion of the working filtrate (e.g., drilling fluid) into the reservoir may result in damages such as clay swelling, dispersion and migration, and water blocking, and have impacts on the accuracy of well logging; especially, it will be more difficult to drain off the drilling fluid if the foreign fluid in the drilling fluid has physical and chemical actions with the reservoir fluid or reservoir rock; consequently, the oil and gas resources in the reservoir may be contaminated.
It is proven in field tests that the above problem can't be solved by conventional drilling fluid systems. Hence, it is especially important to develop new drilling fluid additives and reservoir protective drilling fluid techniques to meet the current challenge of complex geological conditions. Up to now, though long-time researches on reservoir protective drilling fluid techniques have been made in China and foreign countries, and techniques such as shielded temporary plugging technique, fractal geometry-based temporary plugging technique, D90 ideal temporary plugging technique, broad-spectrum temporary plugging technique, Alkali soluble micrometer-level cellulose temporary plugging technique, D50 temporary plugging technique, filming technique, oil film technique, etc., have been developed, the protective effect of these techniques is not ideal for middle permeability reservoirs. Hence, it is urgent task to develop a novel drilling fluid system that has enhanced plugging performance and is suitable for middle permeability reservoirs.
The existing drilling fluids have properties that can essentially meet the requirement for cuttings carrying and well wall stability, but don't provide an ideal reservoir protection effect, and the core permeability recovery value of the reservoir is low. Though temporary plugging agents are added in these drilling fluids, the plugging performance is not high because the structural characteristics of middle permeability reservoirs are not taken into consideration fully. Consequently, the filtrate may intrude into the reservoir and thereby results in damages such as water sensitivity and water blocking, etc.